As 1960s counterculture morphed into the me-decade of the 1970s, part of any hip library was the Illuminatus trilogy, whose co-author, Robert Anton Wilson, has died aged 74. Post-polio syndrome had weakened his legs and a fall confined him to bed. The trilogy - Eye Of The Pyramid, Golden Apple, and Leviathan, all published in 1975 and co-written with Robert Shea, who died in 1994 - grew out of their experience as editors at Playboy, particularly from the Playboy Forum, readers' letters which they answered and occasionally wrote. The steady stream of conspiracy theories they received inspired them to detail the battle of the Bavarian Illuminati, secret controllers of the world, against the Discordians, whose embrace of chaos may have owed more than a little to the paranoid uses of entropy in the fiction of Thomas Pynchon.

Illuminatus brilliantly incorporated elements from the cult literature of the time: borrowing elements of Colin Wilson, Philip K Dick (and his SF pulp predecessors), Flann O'Brien, Carlos Casteneda, Timothy Leary and Kurt Vonnegut in a mix both knowingly tongue-in-cheek and pseudo-intellectually challenging. It was also funny. "My goal," said Wilson, "is to try to get people into a state of generalised agnosticism, not about God alone but agnosticism about everything."

Born in Brooklyn, Wilson contracted polio as a child and felt the effects throughout his life. He studied engineering and mathematics at Brooklyn Polytechnic and then New York University, but engineering soon gave way to sales, then to copywriting and freelance journalism, most notably in Paul Krassner's early counterculture journal The Realist. He was hired as an associate editor at Playboy in 1965, perhaps because of his Realist cover story "Timothy Leary and the Psychological H-Bomb".

Playboy at the time saw itself at the cutting edge of the new liberated lifestyle. It is interesting to see, in the progression of the four books he wrote while working there (Playboy's Book of Fabulous Words, Sex and Drugs: A Journey Beyond Limits, Sex Magicians and The Book of the Beast), a presaging of the concerns of Illuminatus, and the conceptual leap the trilogy made, from consumer lifestyle in the direction of a philosophical world view, no matter how facetious. Published as paperback originals, they were a cult hit. Never bestsellers, they have remained in print ever since. Their biggest impact in this country came when Ken Campbell's Science Fiction Theatre of Liverpool adapted the trilogy for the stage. The 10-hour epic debuted in 1976, then became the first presentation of the National Theatre's Cottesloe in 1977.

Wilson followed up with the autobiographical Cosmic Trigger: Final Secrets of the Illuminati, which included encounters with extraterrestrials while under the influence of peyote and mescaline. He would produce two more volumes of the Cosmic Trigger, in 1991 and 1995. Where he had fun with the conspiracies of Illuminatus, in his non-fiction he pursued the revelation of a parallel kind of secret control, the way society acts to restrain individual consciousness, and the search for freedom through expanding that consciousness. Drugs played an important role. He collaborated with Leary on two books, Neuropolitics (1978) and The Game of Life (1979) reflecting those concerns, but he also practised what he preached.

A prodigious smoker of marijuana, he once told Krassner that he wrote the first draft of each book "straight, the second stoned, then straight, then stoned, and so on, until I'm absolutely delighted with every sentence. Or until irate editors start reminding me about deadlines, whichever comes first." Marijuana also helped with the effects of polio, and as they worsened he became an advocate for its medical use.

His best science fiction was the Schroedinger's Cat trilogy (1980-81), which brought quantum physics into the mix. Illuminatus became a sort of alternative franchise: The Illuminati Papers (1980), Masks Of The Illuminati (1981) and another trilogy, The Historical Illuminatus Chronicles (1982, 85, 91) all followed. His dissertation for a PhD in psychology from the unaccredited Paidea University was published as Prometheus Rising (1983). Other works included a play, Wilhelm Reich In Hell (1987), Quantum Psychology (1990), and Everything Is Under Control: An Encyclopedia Of Conspiracy Theories (1998). Among many projects, all of which generated writing, recordings, websites and followings, were the Church of the Sub-Genius, the Association for Consciousness Exploration and E-Prime, dedicated to the elimination of the verb "to be" from the language in favour of something less definitive.

When his last illness became terminal, he was bombarded with financial support from readers. He paraphrased comedian Jack Benny to thank them, saying: "I do not deserve this, but I also have severe leg problems and I don't deserve them either." His last posting on his website said: "I look forward without dogmatic optimism but without dread. I love you all and I deeply implore you to keep the lasagna flying."

He married the writer Arlen Riley in 1958; she died in 1999. He is survived by a son and two daughters; a third daughter was killed in 1976 during a robbery.

John Higgs writes: When I visited Robert Anton Wilson in December 2004, he looked frail. From photographs I was expecting a stocky, round-faced man, but the Bob I met was thinner in the face, which gave his ever-smiling eyes more prominence. His white beard had grown long and gave him the look of a Taoist sage. His voice was weak but this did not matter, for his mind was sharp and witty and what he said was worth listening to.

In conversation, you realised how liberating his brand of agnosticism is. By not believing in anything he was free to examine everything. To Bob, everything was interesting. This openness was life-affirming because he did not shut himself off from the good and the humour in things. His pleasure in wild ideas may have sidelined him as a contemporary thinker, but his approach was an antidote to fundamentalism. For Bob, fixed belief was intellectual suicide, and the framing of an argument into only two competing sides was absurd. He is gone but, I think, there is still much we will learn from him.

· Robert Anton Wilson, writer, born January 18 1932; died January 11 2007